1. Field of the Invention
The present invention relates to a cyclic polysulfide compound and a method of production thereof as well as a rubber vulcanization agent comprises the same and a rubber composition containing the same.
2. Description of the Related Art
A cross-linked rubber sulfur vulcanized with sulfur has polysulfide bonds, and therefore, is inferior in heat resistance and vulcanization reversion. To solve the problems of heat resistance and vulcanization reversion, it is known that a vulcanization agent such as tetrasulfide polymer or cyclic polysulfide is effective (Noboru Yamazaki et al.: Abstracts of 1981 Research Announcement Conference, p. 53, 2–17, Japanese Rubber Industry Association and Japanese Unexamined Patent Publication (Kokai) No. 10-120788). In particular, a cyclic polysulfide is preferable in terms of the cross-linking efficiency, but the methods of production of a cyclic polysulfide reported up to now are not practical due to problems such as the long production process or use of expensive materials (Japanese Unexamined Patent Publication (Kokai) No. 58-122944).
Japanese Unexamined Patent Publication (Kokai) No. 58-122944 reports the following method of production of a cyclic polysulfide compound: 
According to this production method, it is necessary to react an expensive material, i.e., dithiol, and 2 equivalents of an expensive chlorotrimethylsilane as a protecting group of dithiol. Chlorotrimethylsilane and the S—SiMe3 group are easily hydrolyzed by moisture, and therefore, as a reaction condition, it is necessary to dry or treat the reaction apparatus so as to remove the humidity or moisture. Further, it is necessary to separately synthesize S3Cl2. Furthermore, the yield is 70% or lower, and therefore, there is a problem in the purification of the desired product.